If you want to do some speculative logo work you are better off going to Logosauce LogoSauce - Logo design, inspiration and information.
and join the competitions , you may even win! But what you will see is loads of other peoples interpretations of logo and brief, it will help you gauge what is required out there and how best to improve.
What I've found in the photography world - is that the clients you start out with, doing them a favour, building your portfolio, expanding your client list etc - think is that if you give them work for free initially, then they will always want your work for free. And you become the 'guy that does stuff for free'.
As others say, practice with competitions, build you own website to show off 'made up logos', etc rather than setting the value for your work at zero.
Good to hear you have some clients Mohammad, as the other guys have already mentioned be careful in offering reduced rate services as you may attract the wrong type of client, for the time being I will close this thread.